Many electrical apparatuses incorporate power converters which are switching-based with high switching frequency. Due to fast switching transitions, switching-based operation which is implemented in one apparatus produces electromagnetic interference which is transmitted to the AC network used for supplying power to this apparatus, and also transmitted to other apparatuses. In particular, common mode currents which are produced by one apparatus may cause damages to other apparatuses which are also powered by the same AC network. For example, reliability and lifetime of motors are significantly affected by such common mode currents. Indeed, capacitive couplings exist between the stator and rotor of a motor, so that a shaft voltage which is due to the common mode currents may cause electrical discharges through the motor air gap, which then may damage the insulation barrier. Also mechanical unbalance and wearing of the rotor bearings may be caused by the common mode currents.
Because of these reasons, electromagnetic interference (EMI) standards such as CISPR 11/EN 55011 and CISPR 22/EN 55022 set limits for the common mode currents which are transmitted by an electrical apparatus back to the power-supplying AC network. Plural form is used for the common mode currents due to their magnitudes for all frequency values. Then for meeting the requirements of these standards, some electrical apparatuses have to be equipped with common mode filter devices.
Such common mode filter device is electrically inserted between the apparatus and the AC network, so as to form a barrier against transmission of the common mode currents from the apparatus to the AC network. Because such barrier has to be effective for common mode current for industrial applications, the common mode filter device is commonly comprised of one or several low-pass filters, each low-pass filter being based on at least one capacitor and at least one inductor. For example, such common mode filter device to be connected between the electrical AC network and an electrical apparatus may comprise:                a first electrical connection which is dedicated for connecting an earth terminal of the AC network to a common terminal of the apparatus;        at least two second electrical connections each for connecting a respective phase terminal of the AC network to a respective power terminal of the apparatus; and        separately for each second electrical connection: two LC-filters which are arranged serially, each LC-filter comprising a separate capacitor which is connected between this second electrical connection and the first electrical connection, and each LC-filter also comprising a separate common mode inductor which is arranged in the second electrical connection upstream of the capacitor of the same LC-filter, relative to a direction from the AC network to the apparatus.        
Such common mode filter device is of passive type. It is more efficient for reducing the common mode currents which are produced by the apparatus when each inductor and each capacitor have large values, but the unit price of the common mode filter device is then higher. Also the common mode filter device is then bulky and large, resulting in heavy and cumbersome electrical equipments.
Active filters are also well known. For example, one LC-filter comprised of one inductor and one capacitor connected as indicated above, may be completed with an active circuit which is arranged for sensing the voltage or the current at the filter output, on the AC network side. Then the active circuit is equivalent to a current source which is connected in parallel to the AC network, or to a voltage source which is connected serially between the LC-filter and the AC network. In both cases, the active circuit is designed so that the equivalent current source or voltage source reduces the common mode current which is transmitted back into the AC network. A major advantage of such active common mode filter devices is to allow reducing the values of the inductor and capacitor of the LC-filter while maintaining substantially constant efficiency for reducing the common mode currents. As a consequence, the common mode filter device is reduced in size and price. Such active common mode filter devices are presented in particular in Non Patent Literature 1.
In particular, there are already active common mode filter devices which implement magnetic operation for sensing the common mode currents which are transmitted to the AC network.